Heretofore, for near-field light, a type of point light source for light generation through a hole having a size of not more than the light wavelength as formed at the tip of a metal-coated optical fiber has been energetically studied and developed to support the development of techniques of immersion lithography and near-field microscopy.
Regarding near-field light sources, however, only those of a nanoscale size could be realized in principle, and though the possibility of their application to photochemical reaction with near-field light has been latently suggested, it could not as yet reach realistic application.
For example, Non-Patent Reference 19 describes a case where a light aperture Br having a size of not more than the wavelength of light is formed at the tip FP of an optical fiber coated with a metal Mt, the light Lt having propagated through the core layer Wg of the optical fiber leaks out (as a near-field light), and the near-field light triggers vapor decomposition of two-photon reaction to produce a nanostructure of zinc, as shown in FIG. 24.
Non-Patent Reference 20 describes a case where gold blocks of 100 nm×100 nm are two-dimensionally arrayed via a gap distance of not more than 10 nm to form a gold nanoblock two-dimensional array structure through electron beam lithography, as shown in FIG. 25, and the gold blocks are made to generate near-field light by the use of an external polarized light source to thereby realize a region in which the near-field light is enhanced only in the light polarization direction.
In these cases, two-photon reaction or three-photon reaction is surely verified by the use of a strong near-field light. However, these cases have some drawbacks in that (1) special probe technique or electron beam lithography is employed and therefore the cases are inconvenient in point of the cost and the apparatus operation, (2) in measurement, a scanning probe microscope or the like is needed, and the cases are inconvenient in point of the cost and the apparatus operation, (3) chemical reaction in solution is usual, but in these cases, the reaction is in vapor or is attained by the use of a solid system and the product is solid, and therefore the cases lack versatility, and (3) the product is a nano-scale solid and therefore could not be identified through quantitative analysis such as NMR, etc.
Metal nanoparticles having a particle size of from 1 to 100 nm can generate localized light (hereinafter referred to as near-field light) having a size corresponding to the radius thereof. Whilst ordinary light propagates in air, near-field light propagates along the surface of scatterer such as metal nanoparticles, etc. Accordingly, a metal nanoparticle array structure comprising two-dimensionally arrayed metal nanoarrays formed on a substrate in which the distance between the metal nanoparticles is from 1 to 10 nm can generate a large electric field or an extremely bright near-field light in the gap between the metal nanoparticles. With that, the metal nanoparticle array structure of the type can be utilized as a near-field light two-dimensional array.
As a reaction light source in a microreactor, it may be considered to use a near-field light two-dimensional array that comprises a metal nanoparticle array structure.
Microreactor is an apparatus for generating chemical reaction in a microchannel having a micron-scale space. As compared with any other apparatus where chemical reaction is carried out in a space having a larger scale, the microreactor is excellent in point of the energy efficiency, the reaction speed, the yield, the safety, the scale-up performance, the experiment space, the reaction substrate mixing performance, and the control performance for experiment conditions such as reaction temperature, etc. Microreactor is much used for reaction in a liquid phase and a vapor phase, and there are reports of a combined case with a thin-film photocatalyst or the like. Another advantage of microreactor is that scaling up is easy by increasing the number of the microchannels therein.
When a metal nanoparticle array is arranged in the flow path in such a microreactor, then near-field photoreaction can be realized efficiently on the surface of the metal nanoparticle array and therearound. In other words, near-field photoreaction can be carried out in the microchannel having a micron-scale space, and the probability that the reaction substrate could reach the surface of the metal nanoparticle array can be increased than in the near-field photoreaction in a space having a larger scale, and the reaction efficiency can be thereby enhanced. Taking the diffusion distance between the starting material for photoreaction and the photoreaction product into consideration, nearly 100% near-field photoreaction can be realized in the microchannel having a micron-scale space.
Actually, however, a method of efficiently generating photoreaction by the use of a near-field light in a microreactor is not realized.
For using a metal nanoparticle array structure as the light source of such a near-field light, the size and the shape of the metal nanoparticles and also the distance between them must be uniformly controlled, which, however, is difficult.
Some reports have already been made relating to the technique of producing a metal nanoparticle array structure. For example, nanosphere lithography (Non-Patent References 1 to 3) and electron beam lithography (Non-Patent Reference 4) are already-existing techniques, which, however, have some problems in that the lithography apparatus is expensive and a large-scale structure is difficult to produce.
Regarding the technique of note for fixation on a substrate such as chemical bonding or the like, there are known a thiol bond (Non-Patent References 14 to 15), a CN bond (Non-Patent Reference 16), and a coordination bond (Non-Patent References 17 to 18). According to these methods, however, a metal nanoparticle array structure having a high coverage is not obtained.
Production according to a self-organizing method has been tried. As a method of using an external pressure, there are known a Langmuir method (Non-Patent References 5 to 8), a Langmuir-Blodgett method (Non-Patent References 9 to 10), a dip coating method (Non-Patent Reference 11), use of solid-liquid interface (Patent Reference 1). As a method of using an external field, there are known an electrophoresis method (Non-Patent Reference 13, Patent Reference 3), and a solvent evaporation method (Non-Patent Reference 12, Patent Reference 2).
However, these methods do not have any strong immobilizing means such as chemical bonding or the like between the metal nanoparticle array structure and the immobilizing substrate, and are therefore problematic in that, when the structure is arranged in a microchannel, the metal nanoparticles would readily peel away from the immobilizing substrate as exposed to the solution or the like running through the flow path.